The present invention relates generally to recorded devices, and more particularly to an electrophotographic recorded device. The present invention is suitable, for example, to an electrophotographic printer. The "electrophotographic recorded device", which is typified by a laser printer, denotes a nonimpact printer that toner onto a latent image that has been formed on a photosensitive drum by laser for exposure, and then transfers the toner image onto a printing paper, thereby forming an image. The inventive recorded device is widely applicable not only to a discrete printer, but also to a photocopier, a facsimile unit, a computer system, a word processor, and a combination machine thereof each having a printing function.
An electrophotographic printer is one example of the electrophotographic recorded devices, which has been developed in recent years as a desktop type that provides the easy setup in an office. The electrophotographic printer has characteristics such as an excellent operability, wide range of acceptable media, and cost efficiency. It also provides a high-quality print output, and is thus expected to achieve a higher quality and higher speed of printing.
The electrophotographic printer generally includes a photosensitive drum and a transfer unit having a transfer roller. The printing paper as a recorded medium passes through between the photosensitive drum and the transfer roller. The photosensitive drum is uniformly negatively charged by a pre-charger, and an area on which toner is to be deposited is discharged by laser (exposure to light) to form a latent image. Subsequently, a development device makes charged toner adsorbed into the discharged area through a development roller, and forms a toner image. The transfer unit absorbs and attaches onto the printing paper the toner image formed on the photosensitive drum by electrostatic adsorption using an electric field, thereby transferring the toner image onto the printing paper. The transfer roller polarizes the printing paper, making a surface facing the transfer roller positive, and the other surface facing the photosensitive drum negative.
After the toner is transferred, the printing paper successfully separates from the photosensitive drum using its flexural rigidity and separator electrode. The smaller a radius of curvature of the printing paper is, the more easily it can follow a curved surface, and the thicker the printing paper is, the larger its flexural rigidity becomes. In addition, the separator electrode is provided along a printing paper feed path at a subsequent stage of the transfer roller and at a side of the transfer roller. Therefore the printing paper that has been positively charged at the side of the transfer roller after the toner is transferred is attracted to the negative separator electrode, which promotes its separation from the photosensitive drum.
However, photosensitive drum's increased diameter and/or printing paper's reduced thickness have lowered the flexural rigidity of the printing paper. This has made it difficult to separate a printing paper from the photosensitive drum after the transfer process only by the conventional method using an electric charge generated by the separator electrode, whereby the printing paper has easily followed and jammed the photosensitive drum. In particular, if the separator electrode and the printing paper come into contact with each other, the positively charged transfer-roller side of the printing paper reverses its polarity to a negative pole, and the photosensitive-drum side is polarized into a positive pole. Consequently, the paper is attracted to the negatively charged photosensitive drum, and the foregoing problems would be more likely to occur.